1. Field of the Invention
The present invention relates to an optical loss filter in which a long-period grating is formed on an optical waveguide. As disclosed in U.S. Pat. No. 5,703,978, for example, the long-period grating herein differs from a short-period grating for reflecting light having a predetermined wavelength, and is one which converts, of core-mode light propagating through a core region while being confined there, a predetermined wavelength into cladding-mode light and emits the cladding-mode light out of a cladding region.
2. Related Background Art
An optical waveguide (e.g., optical fiber) in which a refractive index modulation having a period of several hundreds of micrometers (long-period grating) is formed in an optical waveguide region converts, of core-mode light propagating through the optical waveguide while being confined in a core region thereof, a predetermined wavelength into cladding-mode light and emits the cladding-mode light out of a cladding region. Namely, the optical waveguide formed with a long-period grating acts as an optical loss filter having a wavelength selectivity. As can be seen from the loss-generating mechanism mentioned above, this optical loss filter has a characteristic that it yields no reflection, whereby it is favorably used in order to attenuate a predetermined wavelength of core-mode light with no reflection. For example, the optical loss filter is favorably used as a gain equalizer for equalizing the gain of an optical amplifier in wavelength division multiplexing optical communications.
As shown in FIG. 14, an optical loss filter in which a normal long-period grating having a uniform period is formed in a core region of an optical waveguide has the form of a Gaussian like function at a wavelength width of 100 nm, thus yielding only one loss peak. However, the optical loss filter employed as the above-mentioned gain equalizer or the like is required to have a complicated transmission characteristic such as the one shown in FIG. 15. For fulfilling such a requirement, as shown in FIG. 16, a plurality of optical waveguides 83 (three in the drawing) each having excess length parts 82 at both ends of a long-period grating 81 are prepared, the respective excess length parts 82 of adjacent optical waveguides 83 are fusion-spliced to each other while each long-period grating 81 is accommodated in a package 84, and then each of thus fusion-spliced parts 85 is fortified with a reinforcement. The optical loss filter shown in FIG. 16 has such a loss characteristic that the respective loss characteristics of the plurality of long-period gratings are superposed on each other.
The inventors studied the above-mentioned conventional technique and, as a result, have found a problem as follows. Namely, the optical loss filter, shown in FIG. 16, having a configuration in which excess length parts are fusion-spliced to each other becomes so large that it is hard to be accommodated into a package as a whole.
On the other hand, a plurality of long-period gratings may be formed in cascade on a unitary optical waveguide so as to construct an optical loss filter in order to reduce its size to such an extent that it can be accommodated in a package as a whole. In this case, the optical loss filter as a whole can be made smaller since there is no need to provide excess length parts for fusion splicing. However, the loss characteristic of thus configured optical loss filter differs from one in which the respective loss characteristics of a plurality of long-period gratings formed on a unitary optical waveguide are superposed on each other, thus failing to become the desirable one.
In order to overcome the problems mentioned above, it is an object of the present invention to provide an optical loss filter which can reduce its size and can easily realize a desirable loss characteristic.
The optical loss filter according to the present invention comprises a plurality of long-period gratings formed in cascade on a unitary optical waveguide having a core region and a cladding region, wherein cladding-mode light emitting means for emitting cladding-mode light to the outside is provided between two adjacent long-period gratings in the plurality of long-period gratings. In the optical loss filter, the cladding-mode light converted from the core-mode light in a long-period grating is emitted to the outside by the cladding-mode light emitting means, whereby the ratio at which it returns to the core-mode light is lower in the other long-period gratings. Therefore, the loss characteristic of the optical loss filter is one in which the respective loss characteristics of the plurality of long-period gratings are superposed on each other.
In the optical loss filter according to the present invention, the cladding-mode light emitting means can be a coating layer surrounding the cladding region at a part between the two adjacent long-period gratings. In this case, the cladding-mode light converted from the core-mode light in a long-period grating is emitted to the outside by way of the coating layer acting as the cladding-mode light emitting means. The coating layer is preferably a resin. The refractive index of the coating layer is preferably at least 1.25 but not exceeding 1.65, more preferably at least 1.35 but not exceeding 1.55, in a wavelength band in use. If the optical waveguides are made of a silica type material in this case, the refractive index of the coating layer and the refractive index of the cladding region will attain respective values relatively close to each other, whereby the cladding-mode light will be emitted to the outside efficiently. Preferably, the coating layer has a transmittance of at least xe2x88x9210 dB/mm in the wavelength band in use. In this case, transmission characteristics are less likely to be adversely affected by the heating caused by the radiation light absorption in the coating layer.
In the optical loss filter according to the present invention, the cladding-mode light emitting means can be an outer shape changing portion of the cladding region between the two adjacent long-period gratings. In this case, the cladding-mode light converted from the core-mode light in a long-period grating is emitted to the outside at the outer shape changing portion of the cladding region, which is the cladding-mode light emitting means. The outer shape changing portion can be formed either by chemical etching or by melting upon heating.
Preferably, in the optical loss filter according to the present invention, the plurality of long-period gratings in total are formed within the range of 60 mm or less. This configuration is preferable when accommodating the whole optical loss filter into a single package.
Preferably, in the optical loss filter according to the present invention, the cladding-mode emitting means and each of the two adjacent long-period gratings have a distance of at least 1 mm therebetween. Though a desirable loss characteristic can not be obtained if the distance between the coating layer and each of the long-period gratings is on the order of the refractive index modulation period of each long-period grating, the desirable loss characteristic can be obtained if this distance is at least 1 mm.